My computer cant read my hard drive

Hi.
My name is Victoria.
I plugged in my Hard drive to the USB slot..
But my computer wont read it..
not only my computer.. But same with the computers in my school..
I see the light on the drive and i also hear something working in the disk..
BUt.. my computer cant just find it..
My computer does not react at all..
I tried the Tweak Ui.. but i dont understand what im suppose to do..
And nothing works..
Can anyone help me??